Role of Reactive Oxygen Species in PSII Dynamics

Role of Reactive Oxygen Species in PSII Dynamics 

When proteolytic removal of Dl from inactive PSII cannot proceed at a sufficient rate (in vivo under high stress, or in vitro), Dl degradation becomes much slower and a severe inactivation of PSII charge separation and disassembly of the complex occurs. This process is most probably induced by ROS and radicals produced by aberrant PSII photochemistry, since loss of charge separation activity and PSII disassembly has not been observed under anaerobic conditions. So, besides its function in restoring PSII activity, selective Dl replacement may also represent a means of preventing the formation of ROS and their attack on PSII components.  

Proof of ROS-mediated DI protein damage is a decrease in electrophoretic mobility of the original Dl protein band. Most probably it is a consequence of conformational changes in protein secondary stmcture, mainly in the content of the a-helices and P-sheets, as was detected by FTIR spectroscopy in parallel with degradation of the Dl protein.Oxidation of proteins could be confirmed by detection of carbonyl groups that were used as a marker of ROS-mediated protein oxidation.  

The formation of small protein cross-linking products is also caused by ROS-induced oxidation of amino acids in the Dl protein that may lead to subsequent covalent association with other PSII polypeptides. The three most-frequendy-observed cross-linking products could be detected in illuminated or ROS-treated PSII preparations the D1-D2 heterodimer the DTcytochrome b-559 adduct and the D1-CP43 cross-linking product. The first two are formed in an oxygen-dependent manner, both in vivo and in vitro, in illuminated PSII preparations.  

A recent study of D1-D2 heterodimer formation in illuminated spinach leaves has indicated the cross-linking site with the D2 protein between residues 226-244 of the Dl protein.  

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